Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, in the development of such batteries, designs of new electrodes and batteries to improve capacity density and specific energy are mainly studied.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using a liquid electrolyte such as Ni—MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. For these reasons, the lithium secondary batteries have been advantageously used. However, such a lithium secondary battery has disadvantages in that organic electrolytes used therein may cause safety-related problems such as ignition and explosion of the batteries and that processes for manufacturing such a battery are complicated. Recently, lithium-ion polymer batteries have been considered as one of the next-generation batteries since the above disadvantages of the lithium ion batteries are solved. However, the lithium-ion polymer batteries have a relatively lower battery capacity in comparison to the lithium ion batteries, and its discharging capacity is insufficient at low temperature. Thus, it is urgent to solve these disadvantages of the lithium-ion polymer batteries.
Such electrochemical devices have been produced from many companies, and battery safety characteristics are different in the electrochemical devices. Accordingly, it is important to evaluate and ensure the safety of the electrochemical batteries. First of all, malfunction of the electrochemical device should not cause any damage to users. For this purpose, the Safety Regulation strictly regulates ignition and explosion in the electrochemical devices. In the safety characteristics of the electrochemical device, overheating of the electrochemical device may cause thermal runaway, and explosion may occur when a separator is pierced. In particular, a polyolefin-based porous substrate commonly used as a separator of an electrochemical device shows extreme thermal shrinking behavior at a temperature of 100° C. or above due to its inherent characteristics and its manufacturing processes such as elongation, which may cause an electric short circuit between positive and negative electrodes.
In order to solve the above safety-related problems of the electrochemical device, there has been supposed a separator having a porous coating layer formed by coating at least one surface of a porous substrate having a plurality of pores with a mixture of inorganic particles and a binder polymer. For example, Korean Patent Laid-open Patent Publication No. 2007-0019958, Japanese Patent Publication No. 2005-536857, and Japanese Laid-open Patent Publication No. 1999-080395 disclose techniques relating to a separator in which a porous coating layer made of a mixture of inorganic particles and a binder polymer is provided on a porous substrate.
In the separator having the porous coating layer as mentioned above, if a non-woven fabric is used as the porous substrate, a leak current may be generated to deteriorate insulation of the separator. If a loading weight of the porous coating layer is increased in order to prevent the generation of leak current, the thickness of the separator is increased, which is not suitable for implementing a high-capacity battery.
Thus, there is a need to optimally design the non-woven fabric substrate having a porous coating layer so that the generation of leak current may be prevented without increasing a loading weight of the porous coating layer.